Nb3 Ge superconductive films grown with air

ABSTRACT

A superconductive film of Nb 3  Ge is produced by providing within a vacuum chamber a heated substrate and sources of niobium and germanium, reducing the pressure within the chamber to a residual pressure no greater than about 10 -5  mm Hg, introducing air into the resulting evacuated chamber in controlled amounts and vaporizing the niobium and germanium to deposit a film of Nb 3  Ge on the heated substrate.

This is a division, of application Ser. No. 816,482, filed July 18,1977, now U.S. Pat. No. 4,129,166.

This invention relates to the preparation of Nb₃ Ge superconductivefilms having improved properties.

Thin-film superconductive members are receiving increased attention inthe design of high speed, miniaturized computers where thesuperconductive member, usually in thin film form, is disposed between apair of terminals. One of the most useful properties of asuperconductive film is that it have a high critical temperature T_(c).As used herein critical temperature T_(c) is the temperature in degreesKelvin at which the film becomes 50% superconductive, i.e. thetemperature at which it has one half of the normal conductor resistanceexisting at three degrees Kelvin above T_(c). Also, as used herein, thetransition width, expressed in degrees, is the temperature range overwhich the film goes from its normal conducting state to thesuperconducting state, i.e. a transition width of less than one degreeKelvin extends from the temperature in degrees Kelvin at which said filmbecomes 10% superconducting in that it has 90% of its normal conductorresistance existing at three degrees Kelvin above T_(c) to thetemperature in degrees Kelvin at which said film becomes 90%superconducting in that it has 10% of its normal conductor resistanceexisting at three degrees Kelvin above T_(c). The higher the T_(c)relative to the usual operating temperature, the greater are theelectrical properties of the film, i.e. the higher is the current it cancarry as well as the magnetic field it can withstand without quenchingto the normal state.

In the past, it has not been possible to produce consistently a film ofNb₃ Ge composition with a desirable critical temperature T_(c) andtransition width. When by chance such a film has been produced, it hasalways been produced in an insignificant amount, usually only in shortlengths and widths ranging up to about 0.7 centimeter or with surfaceareas less than 0.5 square centimeter. In contrast to the prior art, thepresent process allows the consistent production of Nb₃ Ge films with acritical temperature T_(c) of at least 19° K. Also, the presentdeposited films have a low transition width which is always less thanone degree. In addition, the present film can be produced with surfaceareas which are significantly or substantially larger than that producedby the prior art.

Briefly stated, the present invention is a control method of preparing asuperconducting member, which may be composed of a substrate carrying aNb₃ Ge film or it may be the film itself, i.e. a self-supporting film,having a critical temperature T_(c) of at least 19° K., said criticaltemperature T_(c) being the temperature at which said film becomes 50%superconducting in that it has one half of its normal conductorresistance existing at three degrees Kelvin above T_(c), which comprisesproviding a vacuum chamber with a substrate having at least onesubstantially smooth surface on which said film is to be deposited, saidsubstrate being at least substantially inert under the conditions ofdeposition, providing said chamber with a source of niobium andgermanium, means for monitoring of niobium flux and germanium fluxindividually, providing said chamber with a source of air, protectingsaid substrate surface with a movable protective means which preventsniobium and germanium flux from contracting it, reducing the pressure insaid chamber to produce an evacuated chamber with a residual pressure nogreater than about 10⁻⁵ mm Hg, heating said substrate to a temperatureranging from 750° C. to 1100° C., vaporizing said niobium and germaniumto produce an impinging flux composition ranging from about 1 part toabout 3 parts of niobium to about 1 part of germanium, allowing saidresidual pressure to stabilize at a level no greater than about 10⁻⁵ mmHg, introducing into said evacuated chamber a controlled leak of airwhich raises said stabilized pressure in said chamber at least about 0.1× 10.sup. -5 mm Hg, removing said protective means from said substrate,and impinging said substrate with said niobium and germanium compositionforming an adherent superconductive Nb₃ Ge film thereon ranging incomposition up to about 2 atomic % from stoichiometric composition, saidfilm having at least a thickness sufficient to form a continuous film.

Those skilled in the art will gain a further and better understanding ofthe present invention from the figures accompanying and forming part ofthe specification, in which:

FIG. 1 is a simplified somewhat schematic view of apparatus which can beutilized to carry out the present process.

FIG. 2 is a graphic illustration of the critical temperature, T_(c), vs.incremental increases in pressure made by bleeding in air into theevacuated chamber for a substrate temperature of 950° C. and 1000° C.

Shown in FIG. 1 is substrate 2 mounted in vacuum chamber 3. Sources ofniobium 4 and germanium 5 are placed within chamber 3 which is alsoprovided with vacuum pump 6 and a source of air 7. A movable protectivemeans 8 is used to protect substrate 2 until niobium and germanium areeach vaporized to the levels required for producing the impinging Nb/Gecomposition. Partitioning means 9 prevents Ge flux from source 5 fromstriking monitor 10 and niobium flux from source 4 from striking monitor12 so that monitor 10 reads niobium flux only and monitor 12 readsgermanium flux only. By flux it is meant herein moving species ofniobium or germanium vapors emitted from their respective sources. Dueto the low pressures of the present process, niobium and germanium fluxtravel in straight lines and both strike all areas of the substratesurface on which the present film is to be deposited. Monitor 10 isconnected to controller 11 and is used to determine the flux level ofniobium within the chamber whereas monitor 12 is connected to controller13 and is used to determine the flux level of germanium within thechamber. Controllers 11 and 13 control the heating power to sources 4and 5. An electron gun (not shown) is always used to vaporize niobiumwhereas a number of heating means (not shown) including an electron guncan be used to vaporize germanium 5. Ionization gauge 14 was used tomeasure the total pressure in chamber 3.

The substrate used in the present process is a solid which is inert orat least substantially inert under the conditions of the presentprocess, i.e. under the conditions of deposition. The size and shape ofthe substrate is not critical. It can be flexible or rigid depending onthe particular desired application. For example, the substrate can be inthe form of a tape, foil, wire or plate.

Since niobium is a strong reducing agent, the substrate surface on whichit deposits may be reduced by it in a minor amount, but the resultingreacted flayer or product should not be more than about 1/2 micron inthickness and in such instance, a thicker Nb₃ Ge film must be depositedto compensate for the niobium lost to the substrate surface

Metallic substrates which may diffuse into the present film at elevatedtemperatures are also useful in the present process provided suchdiffusion does not degrade the superconductive properties of the film.However, if such diffusion does degrade the superconductive propertiesof the film, it is preferable to deposit thicker films to insure thepresence of Nb₃ Ge film free of such degrading diffused substrate.Representative of the substrates useful in the present process aresapphire, silicon or oxidized silicon, polycrystalline alumina, niobium,copper, molybdenum, stainless steels and superalloys such as the nickeland/or cobalt-based superalloys.

The substrate surface on which the present film is to be depositedshould be smooth or at least substantially smooth, i.e. it should befree or substantially free of abrupt elevational differences.Preferably, the substrate surface used for deposition is mechanically orchemically polished to produce a smooth surface. Before being placedwithin the vacuum chamber, the substrate should be cleaned of surfaceimpurities, preferably by washing with alcohol or other solvents anddetergents and rinsing off with distilled water. After drying, it isplaced within the vacuum chamber.

Prior to deposition of the present film, the substrate is heated so thatthe substrate surface on which the film is to be deposited is maintainedat a temperature ranging from 750° C. to 1100° C., and preferably fromabout 900° C. to 1000° C. Temperatures lower than 750° C. are notoperable in the present process whereas temperatures higher than 1100°C. are not practical requiring high power and expensive substratematerials. With increasing impingement rates of niobium and germaniumand larger controlled leaks of air correspondingly higher substratetemperatures are necessary. Heating of the substrate can be carried outby a number of conventional means such as radiant heaters or aresistively heated support.

Niobium and germanium, or sources thereof, each contained separately ina boat or crucible are placed within the vacuum chamber. Positioning ofthe niobium and germanium or sources thereof within the chamber is notcritical, but preferably, and as a practical matter, they are positionedclose together and substantially equidistant from the substrate in orderto have a substantially constant flux composition impinging on thesubstrate. The evaporation rates of the niobium and germanium areindividually monitored with conventional means 10, 11, 12 and 13 such asan ionization gauge type or oscillating quartz crystal type evaporationrate monitors.

In the present process the vacuum chamber is provided with a source ofair which can be introduced into the chamber to provide the chamber witha particular incremental pressure of air during deposition of thepresent film. Such a source of air can be provided in a number of wayssuch as by a variable leak valve or other means capable of providing alow controlled leak rate into the chamber.

In carrying out the present process, protective means 8 such as ashutter which is easily movable within the chamber by remote control ispositioned to protect the substrate from impinging niobium andgermanium. Vacuum chamber 3 is then evacuated by conventional means inassociation therewith such as a vacuum pump 6 so that it has a residualpressure no greater than about 10⁻⁵ mm Hg, and generally such residualpressure ranges from about 10⁻⁵ mm Hg to about 10⁻⁷ mm Hg or lower. Thesubstrate is heated to a temperature ranging from 750° C. to 1100° C.and maintained at this temperature during deposition of the film.

Niobium and germanium are then vaporized. The particular rates ofvaporization of niobium and germanium can be controlled by controllingthe temperature at which each is vaporized. Niobium and germanium shouldbe vaporized to produce an impinging flux composition on the substratecomprised of about 1 part to about 3 parts of niobium for about 1 partof germanium, i.e. an impinging Nb/Ge flux ratio ranging from 3/1 to1/1.

The strong gettering ability of evaporating Nb typically drops thesystem pressure from about 10⁻⁵ to about 10⁻⁶ or 10⁻⁷ mm Hg during thepre-deposition period. When the desired impingement flux composition isreached and when the residual pressure becomes stabilized, i.e. when theresidual pressure has dropped and does not fluctuate significantly withfurther vaporization of Nb, a controlled leak of air is introduced intothe chamber. The particular increase in stabilized pressure due to airto be used during the deposition process is determinable empirically,for example, by determining its effect on the critical temperature T_(c)and composition of the resulting deposited film. Specifically, theoperable incremental air pressure range is determined largely bysubstrate temperature and the impinging rates of Nb and Ge withincreasingly larger incremental pressures of air being required byincreasing substrate temperature or increasing impingement rates.

Impinging rates of niobium and germanium can be controlled bycontrolling the temperature of the sources of niobium and germanium,i.e. an increase in temperature increases the rate of vaporization andtherefore impinging rate.

Incremental air pressure significantly higher than 5 × 10⁻⁴ mm Hg is notuseful since it is likely to oxidize the deposited film resulting in asignificantly deteriorating effect on its properties, whereasincremental air pressure significantly lower than 0.1 × 10⁻⁵ mm Hgappears not to be effective in producing the present film having acritical temperature T_(c) of at least about 19° K. and a transitionwidth less than one degree.

Incremental air pressure within the vacuum chamber can be determined bya number of techniques. For example, it can be measured by an ionizationtype pressure gauge, for example, a Bayard-Alpert type of ionizationgauge.

It has been found that not all of the impinging germanium is retained onthe substrate, that is, some germanium desorbs from the substrate duringthe film growth. The factors affecting desorption of germanium from thesubstrate are substrate temperature, air pressure and the impingementrates of germanium flux and niobium flux on the substrate. The rate atwhich germanium desorbs from the substrate increases with increase insubstrate temperature. Increase in air pressure also increases the rateof germanium desorption from the substrate. On the other hand, increasesin the rate of impingement of niobium flux on the substrate decrease therate at which germanium desorbs from the substrate and increases in therate of impingement of germanium flux on the substrate decrease thefraction of germanium desorbing from the substrate. Desorption of excessgermanium from the substrate by reaction with air and increasedsolubility of germanium in the Nb₃ Ge when air is present are believedto be the reasons for the wide niobium to germanium ratios that areoperable in the present process.

The factors which most affect film growth in the present process are theniobium/germanium flux ratio impinging on the substrate, i.e. thespecific Nb/Ge impinging composition, the impingement rates of niobiumand germanium flux on the substrate, substrate temperature andincremental air pressure. An increase in substrate temperature increasesthe rate of germanium desorption thereby lowering the operable Nb/Geimpinging flux ratio or composition range to less than 3. Specifically,with substrate temperatures higher than 1000° C., more germanium isdesorbed from the substrate and the operable niobium/germanium impingingratio tends to decrease and higher air incremental pressures arerequired since air constituents will also desorb more readily and moredesorbing germanium is available to react with the air constituents. Onthe other hand, lower substrate temperatures, i.e. below about 1000° C.,less germanium is desorbed from the substrate and the operable niobiumto germanium impinging composition tends towards about 3 parts ofniobium to about 1 part of germanium and lower air incremental pressuresare required. With a substrate temperature of 1000° C. and anincremental air pressure of about 2.5 × 10⁻⁵ mm Hg, a suitable ratio ofniobium to germanium is about 2.0 to 2.5 with a total impingement rateof 6 Angstroms per second.

The rate of deposition of the present impinging composition may rangefrom about 0.5 Angstrom per second to about 1200 Angstroms per second.Deposition rates below 0.5 Angstrom per second would require too long aperiod of time to grow the present film to be practical, whereas rateshigher than 1200 Angstroms per second would require high power andprovide no significant advantage. For most applications, depositionrates ranging from about 5 Angstroms per second to about 500 Angstromsper second are preferred. The rate at which the present film deposits onthe substrate is affected by the same factors affecting film growth, andspecific deposition rate is determinable empirically by examining thefilm formed at a particular niobium/germanium impinging composition,niobium and germanium impingement rates, substrate temperature andincremental air pressure.

The rate of impingement of niobium on the substrate is the same as itsdeposition rate since niobium is not significantly desorbed from thesubstrate in the present process. However, since germanium isappreciably desorbed from the substrate, the rate of impingement ofgermanium flux on the substrate is usually higher than its depositionrate. The particular rate of germanium impingement used will depend uponthe particular deposition rate required. Impingement rates of niobium orgermanium can be increased by increasing their rates of vaporizationfrom the sources.

The present Nb₃ Ge film must be continuous but can vary in thickness.Its particular thickness depends largely on its application. Generally,the present film ranges in thickness from a film which is sufficientlythick to be continuous to one with a thickness of about 5 microns. Filmsthicker than 5 microns provide no significant advantage. For mostcomputer applications, the thickness of the film ranges from about 1/2micron to 3 microns.

The length and the width of the film or its surface area, can be varied,i.e. it can be produced in a number of configurations. The length of thefilm is limited only by the length of the substrate. The width of thefilm can range up to several centimeters, usually up to about 10centimeters.

The present process can be made continuous by providing a vacuum chamberwith a suitable inlet and outlet means for a movable or continuouslymoving substrate which, for example, may be in the form of a tape, whichremains within the chamber for a period of time sufficient to depositthe present film thereon. In such instance there is no maximumlimitation on the length of the present film.

The present film is of nominal Nb₃ Ge composition by which it is meantthat its composition may vary about 2 atomic % from its stoichiometriccomposition. In addition, the present film is polycrystalline with afine grain size usually less than one micron. It has a T_(c) of at least19° K. and usually higher than 19° K., preferably it has a T_(c) rangingfrom 19° K. up to about 19.9° K., and still more preferably it has aT_(c) higher than 20° K. The present film has a transition width lessthan one degree Kelvin.

If desired, the present film can be removed from the substrate by anumber of techniques, such as, for example, by etching away thesubstrate. In such instances, the film should be thick enough to beself-supporting, i.e. it should be at least about 1 micron in thickness.

The present film, either in self-supporting form or in its formas-deposited on a substrate is useful in computers. In its as-depositedform on a flexible tape it is useful as a conducting member for otheruses such as superconducting magnets.

The invention is further illustrated by the following example where anumber of runs were carried out in substantially the same manner exceptis noted.

EXAMPLE

The apparatus used in this example was substantially the same as setforth in FIG. 1. Each substrate was sapphire, 0.025 inch thick, about11/2 inch long and 3/4 inch wide. Each substrate had a smooth surfacepolished to 1 μ-in. finish for deposition of the film. Each substratewas clamped to a 6 mm thick Mo plate that was heated on the back side bysheathed Ni-Cr wire. The temperature of each substrate was measured by athermocouple embedded in the center of the Mo plate. 3 layers ofradiation shielding with a narrow window for the deposition area, about0.8 cm wide and 3 cm long of the smooth polished surface, were utilized.

A movable shutter operable by remote control was positioned to protectthe deposition area of the substrate from impingement by niobium andgermanium.

Niobium metal and germanium metal were used as sources of niobium andgermanium. Each was placed as closely perpendicular to the substratedeposition area as possible about 25 cms from the substrate, and closetogether separated by partitioning means which prevented germanium fluxfrom striking the monitor for niobium and which prevented niobium fluxfrom striking the monitor for germanium.

The niobium metal was contained in a water cooled copper crucible andwas heated by an electron gun. The germanium metal was contained in aceramic coated tungsten wire basket and was heated by passing anelectrical current through the tungsten wire basket.

Rate monitors of oscillating quartz crystal type were used to measurethe evaporation rates. These gave rate values with approximately ±5%accuracy. Impinging flux composition was controlled by changing theimpingement rates to the substrates by changing the evaporation rate.

From a series of earlier runs wherein the same apparatus conditions wereused, data was produced for given periods of time for particular ratesof evaporation which produced elemental films of Nb or Ge of particularthickness which allowed graphs to be plotted, i.e. evaporation rates vs.rate monitor readings and from these graphs an impingement rate for Nbor Ge could be determined for particular rates of evaporation.

After the vacuum chamber was pumped down to a residual pressure of 1 ×10⁻⁵ mm Hg, the substrate was heated to a temperature of 1000° C. andmaintained at such temperature during the entire deposition process.

Niobium and germanium were then heated to a vaporizing temperature.

The strong gettering ability of evaporating Nb typically dropped thesystem pressure from 10⁻⁵ to 10⁻⁶ and 10⁻⁷ mm Hg during thepre-deposition period. When the desired Nb/Ge impinging flux ratio ofabout 2.5 ±10% was attained using a total impingement rate of about 6Angstroms per second and the resulting residual pressure stabilized, airwas bled into the chamber and when the total pressure in the chamberstabilized at the values indicated in FIG. 2, the shutter was removedfrom the deposition area of the substrate to allow impingement thereonof niobium and germanium flux.

Each run ranged from about 500 to 600 seconds. At the end of each run,the shutter was positioned to protect the substrate deposition area, theair was turned off and heat to the substrate, Nb and Ge was turned offand the substrate was cooled, usually to about 150° C., at which pointthe vacuum pump was turned off and the system was vented with nitrogengas.

Each film deposited on the substrate was examined and found to becontinuous and highly adherent to the substrate.

The critical temperature, T_(c), of each deposited film was uniformthroughout the film within ±0.1° K. and was determined by measuring itsresistance with a 4-point probe in a double wall cryostat that wascooled with liquid helium.

In FIG. 2, the critical temperature T_(c) point plotted is thetemperature at which the film became 50% superconducting in that it hadone half of its normal conductor resistance which existed at threedegrees Kelvin above T_(c).

Each vertical bar indicates transition width and ranges from when thefilm became 10% superconducting in that it had 90% of its normalconductor resistance which existed at three degrees Kelvin above T_(c)to the temperature in degrees Kelvin at which the film became 90%superconducting in that it had 10% of its normal conductor resistancewhich existed at three degrees Kelvin above T_(c).

The results are shown in FIG. 2 which displays the effects of airlevels.

FIG. 2 shows that without air at substrate temperatures of 950° C. and1000° C., films are produced with low critical temperatures, i.e. aT_(c) of about 10° K. and about 7° K. Also, FIG. 2 shows that the runswhich used air but which produced films with a T_(c) below 19° K.utilized an incremental air pressure which was either too high or toolow for the conditions used here, i.e. substrate temperature andimpingement rates.

X-ray diffraction data indicate the presence of single phase Nb₃ Ge inthe high T_(c) films.

The following cited copending patent applications filed of even dateherewith in the name of Raymond A. Sigsbee and assigned to the assigneehereof are, by reference, made part of the disclosure of the presentapplication:

In copending U.S. patent application, Ser. No. 816,481 entitled "Nb₃ GeSemiconductive Films", there is disclosed a superconductive film of Nb₃Ge with a critical temperature T_(c) of at least 20° K. produced byproviding within a vacuum chamber a heated substrate and sources ofniobium and germanium, reducing the pressure within the chamber to aresidual pressure no greater than about 10⁻⁵ mm Hg, introducing oxygeninto the resulting evacuated chamber in controlled amounts andvaporizing the niobium and germanium to deposit a film of Nb₃ Ge on theheated substrate.

In copending U.S. patent application, Ser. No. 816,483 entitled "Nb₃ GeSuperconductive Films Grown With Nitrogen", there is disclosed asuperconductive film of Nb₃ Ge with a critical temperature of T_(c) ofat least 17.5° K. produced by providing within a vacuum chamber a heatedsubstrate and sources of niobium and germanium, reducing the pressurewithin the chamber to a residual pressure no greater than about 10⁻⁵ mmHg, introducing nitrogen into the resulting evacuated chamber incontrolled amounts and vaporizing the niobium and germanium to deposit afilm of Nb₃ Ge on the heated substrate.

What is claimed is:
 1. A superconductive continuous film of Nb₃ Gehaving a surface area of at least one square centimeter ranging incomposition up to about 2 atomic % from stoichiometric compositionhaving a critical temperature T_(c) of at least about 19 degrees Kelvin,said critical temperature T_(c) being the temperature at which said filmbecomes 50% superconducting in that it has one half of its normalconductor resistance existing at three degrees Kelvin above T_(c), saidfilm having a transition width of less than one degree Kelvin extendingfrom the temperature in degrees Kelvin at which said film becomes 10%superconducting in that it has 90% of its normal resistance existing atthree degrees Kelvin above T_(c) to the temperature in degrees Kelvin atwhich said film becomes 90% superconducting in that it has 10% of itsnormal conductor resistance existing at three degrees Kelvin aboveT_(c).
 2. A superconducting film according to claim 1 wherein saidcritical temperature T_(c) ranges from about 19° K. to 19.9° K.
 3. Asuperconductive film according to claim 1 wherein said film ranges inwidth up to about 10 centimeters and wherein there is no maximum limiton its length.
 4. A superconductive film according to claim 1 which isself-supporting.
 5. A superconductive film according to claim 1 which isadherently supported by a substrate.
 6. A superconductive film accordingto claim 1 wherein the substrate is in the form of a tape.